In making color filter arrays, separate layers of different colors must be formed. Frequently these layers are formed using dye as the colorant material which is imbibed into dye receiving layers which must be of a controlled thickness, and a precise amount of dye must be used to achieve the appropriate color. See, for example, commonly assigned U.S. Pat. No. 4,764,670 to Pace et al., U.S. Pat. No. 4,876,167 to Snow et al., and U.S. Pat. No. 5,576,265 to DeBoer et al. which describe such a process. An additional problem associated with this process is that the dye receiving layers swell upon the introduction of dyes, often overlapping one another and also limiting the smallest dimension which can be attainable for use over very small filter elements. In addition, the dyes spread within the dye receiving layer, resulting in loss in edge sharpness of the filter elements. Another problem with this process is that dyes are susceptible to fading on exposure to light.
In another method for making color filter arrays, colorant material is dispersed in a photoactive polymer solution. The dispersion is processed as a typical photoresist would be, with the exception that much greater light intensities are needed for exposure.
One problem encountered in both the use of dispersions and in the use of dyed polymers for making color filter arrays is the difficulty in aligning the different color filter layers for each color, so that they do not overlap in regions where overlaps are not desired.
Turning to FIG. 1, which shows a method for making coatings from evaporated organic colorants, a substrate 102 is positioned adjacent to an aperture mask 104. The aperture mask 104 provides an aperture over a portion of the substrate 102. An organic colorant which is to provide the coating is placed into a source boat 100, which is heated by passing an electric current through it. Alternatively, the boat 100 may be heated by the application of radiant heating from a suitably placed heat source. Upon being heated under reduced pressure, the colorant vaporizes and travels from the source, impinging on mask 105. The portion of colorant vapor which passes through the opening in mask 105 travels along the lines 103, and between those lines, depositing on the substrate 102 and mask 104.
There are a number of problems associated with this technique which involves depositing layers in a partial vacuum and is frequently referred to in the art as physical vapor deposition (PVD). In certain cases, it is difficult to control the thickness and uniformity of the colorant deposited on the substrate. The process of vacuum deposition of the colorant typically requires the use of an appropriate placement of sources or masks or moving substrate fixtures to produce a coating which is uniform. However, the colorant material may deposit on the mask and vacuum fixtures to such a degree that it flakes off, creating undesirable contamination and waste of the colorant and requiring frequent clean-up. In addition, the moving fixtures may generate undesirable particulate materials which may cause contamination of the substrate.
Some other shortcomings in making color filter arrays by the PVD process are the need to use a large source-to-substrate spacing which requires large chambers and large pumps to reach a sufficient vacuum, and the need for masks which cause low-material utilization and build-up on the mask with the concomitant contamination problems. Very specific off-axis source location relative to the substrate, which is sometimes needed for uniform coating, causes very poor material utilization. Still further, source replenishment problems exist for coating multiple substrates in one pump-down. When multiple layers are deposited, the process needs to be carefully monitored for the thickness of layers in the multiple colorant coatings in multiple cycles.
When multiple colored layers are used, the overlaps between the colors are determined by the relative positioning of the patterns formed by or in each. For example, when dispersions of colorants in photoresist materials are used, the pattern for each colored layer is determined by a patternwise exposure to light, followed by development. The precise orientation of the patternwise light exposure determines the orientation of the pattern formed in the colorant. Consequently, the precision of the alignment of the pattern of light which is formed by the photomask used to produce it is critical. Because of this, the alignment of successive patterns is often difficult, and overlaps between colors consequently hard to control. A basic discussion of photolithography is given in “Semiconductor Lithography Principles, Practices, and Materials,” by W. M. Moreau, Plenum Press, New York, 1989.
Consequently, a need exists for a method of producing color filter arrays that overcome the above-described shortcomings.